In order to clarify the tightening properties of threaded joints used in ultrasonic vibration system, effects of form accuracy of bearing surfaces on tightening quality of the joints under ultrasonic vibration in the axial direction are investigated. As the results, it is found that the form accuracy of bearing surface influences the tightening quality of threaded joints under ultrasonic vibration. (1) Concerning form accuracy of bearing surface, the tightening quality is affected by the form of bearing surface as well as the inclination of it to the axis of screw thread. In the case that the form is slightly concave and tightening force is large, critical amplitude Al at separation between bearing surfaces is large, which means the joint has high tightening quality. (2) Concerning relative inclination in combining bearing surfaces, it is affected by the inclination which is caused by the clearance between both threads. In the case that the relative inclination is the same degree as that by the clearance or smaller than that, the value of Al depends greatly on tightening force and it increases with an increase of tightening force.